Alpha 2 u globulin is a rat liver protein whose synthesis is modulated by several hormones. The cDNA for alpha 2u globulin has been cloned in pBR322. This alpha 2u globulin cDNA was used in Southern blot and solution hybridization analysis with rat genomic DNA. Such studies demonstrated the presence of 18-20 alpha 2u globulin genes per haploid complement. Several alpha 2u globulin genes have been isolated from a rat library cloned in lambda phage. These genes are contained on different sized restriction fragments, and are surrounded by distinct flanking sequences. Two of these genes were stably introduced into mouse Ltk minus cells using cotransformation with the gene coding for herpes thymidine kinase. When L cell clones containing several copies of the alpha 2u globulin gene were treated with dexamethasone, it was found that alpha 2u globulin mRNA and protein were produced in response to this hormone. This indicates that the information necessary for hormonal response is contained in the DNA fragment used for transfer. Site-specific mutagenesis, and deletions of specific regions in and around the alpha 2u globulin genes, followed by transfer into L-cells, should help define the DNA sequences responsible for this hormonal modulation.